Julien Guillaumin,     Columbus, Ohio

Indications

The indications for MV in veterinary medicine include (1) ventilatory failure characterized by hypercarbia, with a cutoff of PaCO₂ greater than 60 mm Hg or PvCO₂ greater than 65 mm Hg; (2) hypoxemic failure, with objective cutoffs including a PaO₂ lower than 60 mm Hg or pulse oximetry lower than 90% despite oxygen therapy (FiO₂ ≥ 60%); and (3) increased work of breathing with a risk of respiratory arrest. This last criterion is a subjective assessment of a patient showing evidence of respiratory fatigue or substantial distress, even if the blood gas values are still acceptable. Under these circumstances, delayed intervention is unethical and medically inappropriate.

Prognosis

There are only three large retrospective studies of MV in veterinary medicine (Table 11-1). Another set of four studies researched specific populations of canine patients (see Table 11-1). The prognosis for survival to discharge varies greatly depending on the primary disease process. Survival to discharge is reported to be up to 86% for animals with toxicoses, who are usually younger patients with reversible causes and no pulmonary parenchymal disease; 40% for those with pulmonary contusions; and 11% for post–cardiopulmonary resuscitation (CPR) patients (Hopper, 2007). The overall prognosis ranges from 21% to 71% in dogs and from 15% to 42% in cats (see Table 11-1).

Placing a Patient on the Mechanical Ventilator

Selection of drugs for anesthetic induction and sedation depends on the clinician’s preference and the patient’s status (see Chapter 13). Some unconscious patients may not require drugs, but most will not tolerate MV without sedation. Various options are available. Most clinicians use a combination of benzodiazepine and an opioid and then add propofol, barbiturates, ketamine, or dexmedetomidine as needed. For the benzodiazepines, midazolam is preferred over diazepam because diazepam binds to plastic syringes and infusion lines, can precipitate with other drugs, and is less available. Fentanyl is usually the opioid of choice, at “anesthetic doses” of 6 to 20 µg/kg/hr. Propofol is generally a safe anesthetic but its use generates some concerns, notably involving volume, cost, hyperlipidemia, and bacterial contamination because of the soy-based carrier. Long-term propofol use in cats is discouraged because it can cause Heinz body anemia. Pentobarbital was most widely used (65% to 80%) in veterinary medicine in the 1990s but has become obsolete. Although induction and plane of anesthesia are smooth on barbiturates, tremors and seizures can potentially occur at the time of weaning. Ketamine has some cardiovascular sparing effects and requires small volumes when administered as a constant-rate infusion. Dexmedetomidine has cardiovascular side effects including bradycardia, hypotension, and vasoconstriction but can be used at “microdoses” to enhance analgesia and sedation and to decrease the opioid and propofol requirement in some patients.

Paralytics such as nondepolarizing neuromuscular blocking agents (NMBAs) are controversial in human medicine. NMBAs should only be used as a last resort and the clinician should first attempt to troubleshoot both the machine and the patient when patient-ventilator dyssynchrony (PVD) occurs.

Choosing the Correct Settings of the Mechanical Ventilator

A mechanical ventilator moves gas (a mixture of oxygen and medical air) in and out of the lungs. All modern mechanical ventilators follow the equation of motion that links machine characteristics that can be manipulated and patient characteristics that depend on the primary disease and can change over time:

The clinician enters variables into the machine to program the breath that will be delivered. With a pressure-controlled breath, the machine delivers a breath up to a specific airway pressure. With a volume-controlled breath (technically flow-controlled), the machine delivers a certain volume in a given inspiratory time by controlling the flow of gas. Also, the clinician may choose to provide positive end-expiratory pressure (PEEP), in which the baseline pressure at the end of expiration remains supra-atmospheric. This helps to prevent alveolar collapse and recruit alveolar units.

The ventilator can generate different types of breath. In mandatory breaths, the ventilator determines either the delivery and/or the end of inspiration. Spontaneous breaths are initiated and terminated by the patient. Mandatory breath can be assisted, with the patient triggering the breath that is then delivered and terminated by the machine, or controlled, with the system triggering, delivering, and terminating the breath. This is called assist/control (A/C) or continuous mandatory ventilation (CMV).

The ventilator can support a spontaneous breath by adding some positive pressure to “support” its tidal volume. With synchronized intermittent mandatory ventilation (SIMV), both spontaneous and mandatory breaths can be delivered in a synchronized manner, with the machine triggering, delivering, and cycling the breath if the patient does not. Newer modes, such as airway pressure release ventilation (APRV), are more frequently used for humans than for animals.

When MV is initiated, the goal is to achieve the least aggressive settings to maintain adequate oxygenation (PaO₂ 80 to 100 mm Hg) and ventilation (PaCO₂ 35 to 45 mm Hg). The general guidelines for initial settings in animals are:

Inspiratory pressure, respiratory rate, and PEEP can be manipulated to maintain acceptable oxygenation and ventilation. Peak inspiratory pressures up to 40 to 60 cm H₂O and PEEP up to 20 cm H₂O have been used to stabilize patients in severe respiratory failure.

Patient Care

Long-term ventilation can be associated with several nursing care complications, including decubitus ulcers, peripheral edema, corneal ulceration, and oral ranula. These complications occur in at least 5% to 10% of MV veterinary patients. Well-padded tables, regular repositioning, and passive range of motion should be performed every 4 to 6 hours. Ocular ulcer prevention should be done by regular application of artificial tears. Oral care is an important part of the nursing of the MV patient. Canine patients on MV appear prone to ranula development and tongue swelling. Oral care with saline, diluted chlorhexidine solution, a commercial oral rinse, and/or protection with glycerin-soaked gauze is important.

Providing early nutrition is essential for the ventilated patient. Ventilated patients can be fed enterally or parenterally. The combination of central parenteral nutrition and enteral nutrition can provide the patient’s full caloric requirement and promote enterocyte health (see Chapter 7). Enteral nutrition may also decrease the risk of bacterial translocation. When enteral feeding is instituted, residual gastric contents should be monitored and a promotility agent added.

Tracheal intubation should be done using a sterile endotracheal (ET) tube and aseptic technique. A low-pressure, high-volume, cuffed polyvinyl chloride ET tube is the most commonly used tube in veterinary medicine. Less rigid silicone ET tubes are also available. Rubber ET tubes are not recommended. The ET tube should be suctioned and changed whenever necessary.

With a tracheostomy, MV patients often can tolerate the machine with mild sedation, eliminating the need for anesthesia. Tracheostomy also avoids complications associated with the ET tube including ranula and macroglossia. The reported tracheostomy rate in veterinary medicine is 20% to 30%, a number similar to that for human patients, but is increased to 70% for patients in ventilatory failure. A high incidence of complications is reported in cats with tracheostomy, so appropriate warning and proper care are warranted (Box 11-1).